The present invention relates to a pressure fixing apparatus for fixing toner images onto copying paper after the tone images have been formed on the copying paper, e.g., after either the electrostatic latent images produced directly on a copying paper have been developed with toner, or after electrostatic latent images initially produced on a photoconductive member of an electrophotographic copying apparatus have been transferred to a copying paper and then treated with toner. More particularly, the present invention relates to a pressure fixing apparatus in which copying paper with such toner images formed on the surface thereof is passed between a pair of press rollers which are in pressing contact with each other to fix the toner images to the copying paper by pressure exerted by the rollers.
Generally speaking, pressure fixing apparatus are advantageous over heat fixing apparatus because they are usable immediately after the electrophotographic copying apparatus (or like mechanical apparatus incorporating the fixing apparatus) is initiated into operation, whereas heat fixing apparatus require a warm up waiting time of several minutes. Moreover, pressure fixing apparatus have the advantage of being free of fire hazards when the fixing unit is jammed with paper or when some other malfunction occurs.
Pressure fixing apparatus have therefore been widely used especially in electrophotographic copying apparatus. The press rollers used in such known fixing apparatus are produced, for example, by first subjecting the surface of a steel roller to induction hardening, then coating the surface with a hard chromium plating and finally polishing the plating to a specular surface of about 0.5 S, i.e., so as to render the press roller free from any offset when brought into contact with the toner image under high pressure. However, the pressure applied by the specular roller surface to the toner images imparts a high gloss to the fixed images, such that the images have a very poor visibility when seen at certain angles. Additionally, such a glossy copy is not desirable to many people. Thus, the gloss attendant on pressure fixing operation is the most serious drawback of the known apparatus of this type.
To overcome the problem of gloss, it is known to utilize a heater subsequent to the pressure fixing apparatus, such that the toner images fixed by the apparatus are heated and thereby made glossless (see, for example, Japanese Laid-Open Patent Application No. Sho 52-78441 and Japanese Laid-Open Utility Model Application No. Sho 52-63045). Although the heater useful for this purpose may be of smaller capacity than those adapted for the heat fixing of toner images, the use of the heater, as in heat fixing, renders the copying apparatus complex and expensive and involves fire hazards. Furthermore, the user must wait for some time for the warming-up of the heater after closing the main switch. These drawbacks nullify the advantages of such pressure fixing apparatus when substituted for the heat fixing apparatus.
In an attempt to overcome the foregoing drawbacks inherent in the matting of toner images by heating, especially to eliminate the necessity for the waiting time, another apparatus has been proposed which incorporates a rough-surfaced roller disposed subsequent to the pressure fixing apparatus for matting the fixed toner images (see Japanese Laid-Open Patent Application No. Sho 52-129525). However, the matting means provided for the pressure fixing apparatus similarly makes the copying apparatus complex and expensive. In addition, the matting roller, although adapted to act on the toner image after fixing, still involves the problem of offset, i.e., because the smooth glossy face of the image is roughened with the uneven roller surface. The rough roller surface is further subject to losing its matting function due to being smoothened by the pressing and frictional contact with the opposed roller surface. Thus such a proposed apparatus is not practical because of the problems encountered with respect to offset, cleaning of the roller and durability.
An object of the present invention therefore is to provide a pressure fixing apparatus capable of producing glossless or mat-fixed toner images without the necessity of using any special means for matting the fixed images.
Another object of the present invention is to provide a pressure fixing apparatus for producing glossless, fixed toner images without entailing any offset.
Another object of the present invention is to provide a pressure fixing apparatus including press rollers which can be cleaned with ease so as to be free from offset.
Still another object of the present invention is to provide a pressure fixing apparatus which is capable of giving glossless fixed toner images over a prolonged period of time free of any trouble.